Allow you to call new_graph() twice in a row
Allow you to call new_graph() twice in a row


Former-commit-id: 7b0729a6315552e950a2f66726ca56bf406ff6cf
shuheik/dynet,danielhers/dynet,clab/cnn,xunzhang/dynet,clab/cnn,shuheik/dynet,danielhers/dynet,danielhers/dynet,chunyang-wen/dynet,chunyang-wen/dynet,danielhers/dynet,danielhers/dynet,clab/dynet,clab/dynet,clab/dynet,xunzhang/dynet,shuheik/dynet,chunyang-wen/dynet,chunyang-wen/dynet,danielhers/dynet,clab/dynet,shuheik/dynet,clab/dynet,clab/dynet,clab/cnn,xunzhang/dynet,shuheik/dynet,shuheik/dynet,danielhers/dynet,xunzhang/dynet,shuheik/dynet,clab/dynet